1. Field of the Invention
The present invention relates to induction heating for a thermal roller having a roller jacket made of a ferromagnetic material and an inductor spool inside the roller jacket for low-loss transmission and adjustment suitable for processing of the heat output through generation of eddy currents of uniform density in totality or in targeted zones of the outer surface of the roller jacket.
2. Description of the Prior Art
Thermal rollers of the this type consist of a steel cylinder swivel-mounted on front-facing axial flanges. With inductive heating of these rollers the heat is generated directly in the jacket of the hollow cylinder by means of a magnetic alternating field, for which purpose the jacket comprises a material which is sufficiently conductive both electrically and magnetically.
A plurality of inductive heating arrangements for thermos rollers of this kind is known, which utilize induction spools or induction loops of various designs for generating the magnetic alternating field in the roller jacket. They are distinguished essentially by the position and direction of the ampere-turn axis of the induction spools or induction loops relative to the roller jacket or by the direction of the magnetic flow and of the induced eddy current in the roller jacket.
Thus, according to DE 19 53 20 44, an induction roller is known which primarily comprises an induction spool on an iron core in the interior of the roller jacket, of which the ampere-turn axis coincides with the roller axis. The magnetic circuit, in which the magnetic flow develops, essentially comprises the iron core of the induction spool and the ferromagnetic roller jacket, as well as the non-ferromagnetic interstice between the said iron core and roller jacket, forming the so-called air gap of the magnetic circuit.
The magnetic flow generated by the induction spool leaves its iron core, fanning out in the air gap and from there radially entering the roller jacket, where it is bundled in the axial direction before fanning out again in the air gap on exceeding the axial center of the induction spool, and thence entering the iron core again from the other side.
The eddy currents caused by the alternating field in the roller jacket current in a peripheral direction on paths concentric to the roller axis. The eddy current density and the associated heat source density are therefore constant in a peripheral direction. Both dimensions are modified in an axial direction, however, according to the change in the alternating current in the roller jacket as a result of its bundling out of or fanning out into the air gap. For this reason, eddy current density and heat source density in the roller jacket decrease towards its ends from the point located radially over the axial center of the induction spool.
For the purpose of achieving the desired uniform distribution of temperature in an axial direction on the roller surface despite this, in accordance with the known arrangement sealed heat pipes are provided in axial bores of the roller jacket. The heat pipes contain a heat transfer medium simmering in the vicinity of the operating temperature, which brings about a heat and temperature equalization between the center and the ends of the roller jacket on the way to evaporation, convection and condensation.
The manufacture of such axial boreholes in the roller jacket is a very expensive manufacturing process. Moreover, temperature equalization cannot be achieved right into the region of the axis flange in this way.
For this very reason, supplementary auxiliary induction spools are provided in the case of the known induction heat roller in the region of the axis flange. The current generated by the auxiliary induction spools enters the axis flange, where it leads to the additional heating required for complete temperature equalization.
Feeding a correspondingly higher calorific output into the windings of the auxiliary induction spools should prevent any radiation of heat into the unheated regions of the axis flange and into the roller frame during the heating process, thus reducing the time period required for heating up the roller to the level of operating temperature.
An essential drawback to this known arrangement is that it does not permit the development of axial zones of controllable heat output on the thermal roller, in particular in the edge regions of the roller body. This effectively restricts the roller in its usefulness to a predetermined width of the goods webs to be processed and thus to a very narrow range of products. The result is that this may lead to a low level of use of the machine's capacity, which in turn means a low return on capital investment.
One known method for achieving uniform current, eddy current and heat source density in the axial direction and developing axial zones of controllable heat output consists of arranging several induction spools axially adjacent to one another.
According to DE 19538261, each of the induction spools arranged axially adjacent to one another is embedded in an iron core having a U-shaped longitudinal section and terminals of its own.
The U-shaped iron cores and the ends of their flange-shaped legs together form a defined air gap against the inner surface of the roller jacket.
Because of their arrangement with appropriate dimensioning, these magnetic circuits formed by the iron cores and the roller jacket do not permit the current to bundle out from or fan out into the air gap, such that, with the exception of the borderline zones between the individual magnetic circuits, an almost constant current, eddy current and heat source density can be achieved along the roller surface in the axial direction.
This type of generation of the magnetic flow consumes a great deal of energy. When n induction spools are arranged along the roller jacket, the consequence of the smaller air gap width is that the magnetic resistance of a magnetic circuit amounts to approximately the n.sup.th multiple and thus the required exciter output is at least the n.sup.2 multiple, while the overall exciter output is accordingly more than the n.sup.3 multiple of a comparable roller having only one field spool.
The exciter output is converted fully into heat in the induction spool.
To avoid excessive heating of the induction spools, a cooling pipe is provided for a comparable, inductively heated roller, which draws off the heat generated in the induction spools, as in EP 0511549, for instance. This heat is lost to the roller heating, the result of which is a considerable reduction in thermal efficiency.
Another disadvantage of this arrangement is the necessity to monitor and control each of the individual induction spools separately with respect to their heat output, which leads to a very expensive power supply consisting of several independent circuits.
Apart from the fact that additional power losses are caused thereby, such a power supply plant is more expensive and naturally more susceptible to breakdown and accordingly requires continual monitoring during operation.
Particularly low power losses and high thermal efficiency of the inductive heating can be achieved with a solution as disclosed in DE 3416353. This solution contains a ferromagnetic core that fully encases the roller jacket on a peripheral point both inside and outside, which is provided with an exciting coil on its outer limb.
Since the magnetic circuit thus formed has no air gap, the exciter output required for the generation of the magnetic flow is very low. The uniformity of the eddy current and heat source density in an axial direction is very good because of a barely present fanning out of the current in the space between the parallel ferromagnetic limbs of the core.
This solution does not permit the development of axial heat zones. Moreover, a customary coaxial drive is not possible, because the iron core partially covers the roller jacket at its front ends.
In addition, inductive thermal arrangements for rollers are known that have a stationary inductor within the roller. Thus for example DE OS 3033482 describes an inductive heating with an inductor of this kind, consisting of several poles axially neighboring in section and arrayed peripherally in a star formation on an axial through support. Each pole in each section is equipped with one induction winding, so that all the poles of the inductor are electromagnetically active or activatable. The ampere-turn axes of the induction spools are set radially, so that the air gap of the magnetic circuit is located between the ends of the pole and the inner surface of the roller jacket.
The roller jacket constitutes the return yoke of the magnetic circuit between the pole cores of originally neighboring induction spools in a radially opposite direction of ampere-turns. This generates a magnetic field in the peripheral direction in the roller jacket, which surrounds the roller axis between poles with an opposite direction of ampere-turns in segments of a circle with an alternating current direction.
The eddy current induced by the magnetic field flows substantially into a thin layer on the inner and outer surfaces of the roller jacket in alternatively opposed axially directions, so that a longitudinally extended current path in the shape of a torus or of several segments of a torus with a cross section approximating to 90.degree. is formed, whose common axis coincides with the axis of the roller.
In this solution, the thermal sources are situated substantially on the inner and outer surfaces of the roller jacket. Their distributions in the axial direction, especially the progressive heating, is easily controlled by suitably exciting the induction spools of axially neighboring sections. Similarly, it is also possible to control the thermal source distribution and the relative progressive heating in the peripheral direction by suitable staggered excitement of the neighboring peripheral induction spools of the pole star and/or by suitably staggering the air gap between the ends of the pole cores and the inner surfaces of the roller jacket along the roller circumference.
One disadvantage of this and comparable known arrangements is the high cost of material and processing for producing the inductor, especially the induction spools, and the high energy consumption necessary for generating the magnetic field, a consequence of the large volume of their winding, which energy is lost to the heating of the roller surface.
Also thermal sources located on the inner surface of the roller jacket are only partially available for heating the external roller surfaces and transferring the heat to the goods web and then only after a delay.
Finally, the heat radiation to the axis flanges and the load-bearing shaft ends cannot be suppressed effectively enough, as the space available in the axis flange is generally insufficient to house an inductor pole star with the thermal output necessary for thermal compensation.
An arrangement is known from DE OS 4410675 as one possible solution to this problem, wherein the roller is equipped with a resistance heating that can be switched on and off, situated in a hollow volume in the axis flange.
For the purpose of generating a magnetic field shaped at least like the arc of a circle in the peripheral direction in the roller jacket, arrangements are also known in which the inductor spools are situated on the outer circumference of the roller.
A solution of this kind was disclosed in DE 3340683 for example. This arrangement consists of U-shaped pole shoe devices, the ends of whose magnetic limbs are set opposite the external roller jacket surface at a certain distance, which constitutes the non-ferromagnetic air gap in which the roller jacket forms the return yoke. Each pole shoe device has an induction spool. Several pole shoe devices are arrayed axially directly one next to the other and constitute a row of pole shoes that covers the roller externally along the entire roller length requiring heating.
It is possible to array several such rows of pole shoes one next to the other in the peripheral direction, so that the magnetic limbs of neighboring rows are displaced axially to each other.
Nevertheless, this does not eliminate the disadvantages of similar arrangements with an inductor arrayed inside the roller. It is only easier to compensate for the heat radiation at the end of roller with an inductor situated externally, as the flange area can be covered inductively to greater effect in this way.
A further arrangement of this kind is intended to reduce the processing and control expenditure and the related material and energy costs for installing and maintaining a defined axial distribution of eddy current and heat source density, as in the case of the one known from DE OS 4011825. In the solution described here, the inductor is a conductive loop arrayed radially over the surface of the roller, whose current-bearing length can be adjusted by means of conductive, axially displaceable contact bridges between its limbs.
The drawback with this arrangement is that there is no external magnetic conductor of the kind necessary for a sufficiently close inductive coupling of the conductive loop to the magnet jacket. The result is that only a narrow thermal area is created in the immediate vicinity of the conductive loop, of such a kind that its limbs only throw a "thermal shadow" onto the roller surface.
The same shortcoming is found in a comparable inductive thermal arrangement for rollers, disclosed in EP 067 99 61, which also consists of looped conductors over the external surface of the roller. A series of conductive loops forms a conductive loop spiral and is embedded in an envelope in a stationary position over the roller, consisting of a magnetically non-conductive, electrically insulating material. Apart from the fact that, in the absence of the magnetic return conductor, the inductive coupling between the conductive loops and the roller jacket is only weak, the ampere-turns decrease significantly from the center of the conductive spool spiral to its edges, so that it is possible to achieve a constant distribution of flow density and eddy current or of thermal source density neither in the peripheral nor in the axial direction.